An increase of the amount of stored data poses a problem that management of a desired storage place of data becomes complicated and complex. In recent years, full text search service and meta data search service have become utilized against such a problem.
In the full text search service, a search server analyzes data such as files stored in a computer system and previously creates a search index. A user can transmit a search query for searching desired data to the search server and access object data on the basis of a result of the search.
In the meta data search service, data each having a set of a meta data name and a meta data value included in search object data is extracted and search indexes of them are previously created. A user can acquire a search result by specifying a search condition concerning the meta data name and the meta data value to a search server.
Inclusive of use and utilization of big data, the amount of data handled in a computer system tends to increase with increasing speed in recent years. It is predicted that search service of a file or the like will become more and more important.
In general, if the amount of data to be stored increases, the storage cost also increases. As a method for decreasing the storage cost, for example, a storage system in which a storage cost per volume is virtually reduced by combining an expensive high-performance upper-hierarchy storage with an inexpensive low-performance lower-hierarchy storage is disclosed in Patent Literature 1. In other words, a technique of a hierarchical storage is disclosed.
In the hierarchical storage, a hierarchical control server stores a file into a suitable storage on the basis of a utilization situation of stored files. For example, the hierarchical control server first stores a file into an upper-hierarchy storage. Thereafter, the hierarchical control server causes migration of data for which an access frequency is low to a lower-hierarchy storage.